Usually, the methods of preparing resist patterns in the manufacture of semiconductor integrated circuits make use of photolithography, and with rise of the demand for increased density and degree of integration of semi-conductor integrated circuits in recent years lithography using ionizing radiation such as electron beams and X-ray has been attracting attention as the resist pattern preparation method capable of meeting the above demand. As the resist material used for the lithography adopting such ionizing radiation, positive resist materials capable of being degraded into lower molecular weight compounds by irradiation with ionizing radiation, for instance polymethyl methacrylate and polybutene-1-sulfon, are well known in the art. However, such resist materials are inadequate as the resist material for use in the preparation of resist patterns because they have intrinsic drawbacks. That is, polybutene-1-sulfon, although its sensitivity is high, more than ten times that of polymethyl methacrylate, is inferior in resolution to polymethyl mechacrylate. Besides, its adhesion to semiconductor substrates is inferior. Further, it is easily thermally decomposed and thus inferior in resistance to dry etching, and thus it is not compatible with the dry etching process, which is indispensable for high density fabrication. Polymethyl methacrylate is superior in resolution, but its sensitivity is insufficient (for instance, on the order of one-tenth to one-thousandth of the sensitivity of a negative resist crosslinked by irradiation with ionizing radiation). Therefore, for selectively making the irradiated portion soluble it is necessary to increase the exposed dose or extend the period of irradiation, which is disadvantageous from the standpoints of utility and productivity.
For improving the sensitivity of polymethyl methacrylate there have been such attempts as polymerization to high molecular weight, copolymerization with radiation-sensitive materials, and substitution of other degradable ester groups for the methyl ester group. However, the resists with higher molecular weight have inferior coating properties, so that in this case it is difficult to coat a resist layer of a uniform thickness on a substrate. Those which are obtained by copolymerizing polymethyl mechacrylate with radiation-sensitive materials and also those which are obtained by substituting other degradable ester groups for the methyl ester group of polymethyl methacrylate are inferior in thermal senstivity to polymethyl methacrylate and are therefore low in resolution and can not be compatible with the dry etching process.